migration: Improve item on untested selection types.

The first sentence was a bit hard to comprehend since it is not the
selection types that are migrated. Also, let's call the unusual
selection types "experimental features".

manuals/en/main/migration.tex

@@ -141,9 +141,10 @@ \section{Important Migration Considerations}
      referenced in the Migration Job to define the Pool into which the
      data will be migrated.
 
-\item Only migration of Selection Types of Job and Volume have
-     been carefully tested. All the other migration methods (time,
-     occupancy, smallest, oldest, ...) need additional testing.
+\item Migration has only be tested carefully for the "Job" and "Volume"
+      selection types. All other selection types (time, occupancy,
+      smallest, oldest, ...) are experimental features.
+
 \end{itemize}
 
 \section{Configure Copy or Migration Jobs}